In the field of optical telecommunications lasers are used to generate optically encoded signals that are transmitted along optical fibres, typically at wavelengths corresponding to channels of the International Telecommunications Union (ITU) grid. The lasers commonly used are monolithically integrated semiconductor lasers.
Different types of semiconductor lasers are commercially available. Distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers with very narrow tuning ranges may be used to provide single channel operation at a substantially fixed wavelength. Historically it has been common to use separate devices to operate at each channel, which has required a discrete device to be installed in a network for each possible channel, making the installation large in size and expensive, whilst also commonly resulting in a significant level of redundancy, since typically not all available channels will be used.
Rather than using separate devices to operate at each channel it is known to produce a single device that is capable of operating across a range of channels by means of monolithically integrating an array of different fixed or narrow tuning range DBR lasers with a passive optical coupler. Such a prior art device is disclosed in US 2005/0244994, which teaches a plurality of passively integrated DBR lasers, each of which comprises a gain section and a DBR. FIG. 1 illustrates schematically an array 100 of two monolithically integrated DBR lasers 102, 104. Each DBR laser 102, 104 comprises an optical waveguide 106, 108 with a front DBR 110, 112 and rear DBR 114, 116. The front DBRs 110, 112 and rear DBRs 114, 116 of each DBR laser 102, 104 bound respective gain sections 118, 120 and phase control sections 122, 124. The DBRs comprise diffraction gratings, each grating being formed by a corrugated surface between layers of semiconductor material with different refractive indices. The front DBRs 110, 112 are designed to be partially transmissive. The rear DBRs 114, 116 are typically more highly reflective than the front DBRs 110, 112.
The optical cavity of the laser is the region of the device within which light is generated and at least partially reflected typically between two cavity ends, so as to create optical feedback and amplified stimulated emission of light. In operation, a lasing cavity is formed between the front 110, 112 and rear DBRs 114, 116 of each DBR laser 102, 104, and laser light is emitted through the partially transmissive front DBR. The output waveguides 126, 128 from the array 100 of DBR lasers 102, 104 are passively coupled to a common output waveguide 130 by means of an optical coupler 132. The DBR lasers operate with mutual independence.
Disadvantageously, for a wide range of operating channels, monolithically integrated arrays of independent DBR lasers require large laser chips, which are not desirable within size limited optical telecommunications packages. Further, the complexity of such chips will typically reduce the possible manufacturing yield, and the complexity of electrical control required to operate the chip may significantly increase the cost of the associated control system.
In the telecommunications industry it is preferable to use lasers with broad tuning ranges, which are capable of operating across a range of telecommunications channels, which can reduce the inventory requirements for the operators of telecommunications systems. Widely tunable DBR lasers are available that provide wavelength tuning across a broad wavelength range, for example covering the whole of the C- or L-band telecommunications bands, corresponding to wavelengths of approximately 1530 to 1565 nm or 1565 to 1610 nm respectively.
A commercially available design of widely tunable DBR laser is disclosed in U.S. Pat. No. 6,728,279. FIG. 2 schematically illustrates a functionally similar DBR laser 200 having a branched optical waveguide 201 comprising a stem waveguide section 202 and two branch waveguide sections 204, 206. The stem waveguide section 202 has a stem optical reflector 208, provided by reflection from a facet of the semiconductor chip on which the laser 200 is formed. A composite optical reflector is provided by the combined reflections of two tunable branch optical reflectors 210, 212 on the branch waveguide sections 204, 206, which are provided by DBRs. The stem optical reflector 208 and the composite optical reflector 210, 212 bound a gain section 214 and a phase control section 216. The branch waveguide sections 204, 206 are optically coupled to the stem optical reflector 208 by means of a passive 1×2 optical coupler 218 (also known as an optical splitter or recombiner) which has a fixed splitting ratio for light passing through the coupler towards the composite optical reflector 210, 212. The splitting ratio is typically wavelength dependent. The branch waveguide sections 204, 206 of the laser's branched optical waveguide 201 may be regarded as being optically in parallel. Light from one branch waveguide section 204, 206 may not couple directly into the other through the optical coupler 218, and can reach the other only by reflection from beyond the optical coupler.
Each of the two tunable branch optical reflectors 210, 212 produces a comb-like spectrum of reflective peaks that are uniformly distributed in wavelength across approximately the same wavelength range, corresponding with the tuning range of the DBR laser, and the reflective peaks of the two reflection spectra are differently spaced. The reflection spectra of the DBRs may be independently tuned in wavelength, by means of respective electrodes and drive circuitry. In operation, the gain section 214 emits a broad spectrum of light, which is at least partially reflected by the stem optical reflector 208 across a broad wavelength range and by the composite optical reflector 210, 212 at wavelengths corresponding with the peaks of the comb-like reflection spectra. A spectral peak from each of the comb-like reflection spectra may be tuned to become coincident in a chosen wavelength, forming a reinforced spectral peak in the reflection spectrum of the composite rear reflector 210, 212.
In operation, it is only at the wavelength of the reinforced spectral peak, at which the optical feedback of the laser cavity is greatest, that the optical gain produced by the light emitting gain section 214 is sufficient to overcome the optical loss in the laser cavity. This results in lasing at that wavelength and emission of laser light through the stem optical reflector 208. Thus, in operation, both branch waveguide sections 204, 206 of the branched optical waveguide 201 always form part of the lasing cavity, and so in this design of laser 200 the lasing cavity is always branched in the same way as the branched optical waveguide.
Coarse tuning of the laser emission over a broad range of wavelengths may be provided by relative tuning of the reflective peaks of the comb-like reflection spectra of the two DBRs 210, 212. The system of relative adjustment of two scales with differently spaced pitches was devised by Pierre Vernier, and this method of tuning a DBR laser is referred to as Vernier tuning.
Other known designs of Vernier tuned DBR laser include those disclosed in U.S. Pat. No. 4,896,325 and US 2004/0240490. These documents disclose lasers with unbranched optical waveguides, which use different geometries of reflectors and DBRs from that illustrated in FIG. 2.
In a Vernier tuned DBR laser, the non-lasing reflective peaks in the comb-like reflection spectrum of one tunable branch optical reflector 210, 212 must be sufficiently different in wavelength from the non-lasing peaks in the comb-like reflection spectrum of the other tunable branch optical reflector to ensure that there is no risk of significant mode competition with the lasing mode, due to complete or partial overlapping of other reflective peaks. This condition provides a fundamental limitation upon the breadth of tuning range that is achievable with the Vernier tuning method.
A further design of widely tunable DBR laser with an unbranched optical waveguide is described in U.S. Pat. No. 7,145,923, and a schematic cross-section is illustrated in FIG. 3. The tuning mechanism of the laser 300 is different to those described previously. The laser 300 comprises a rear optical reflector comprising a first DBR 302 and a front optical reflector comprising a second DBR 304, a gain section 306 and a phase control section 308. A first DBR 302 produces a comb-like reflection spectrum of peaks, and the second DBR 304 produces a relatively uniform broadband reflection spectrum, in its off state (i.e. not tuned). In operation, a portion of the reflection spectrum of the second DBR 304 is tuned to overlap with another portion, to produce a broad reinforced reflective peak. When the broad reinforced reflective peak and one of the reflective peaks in the spectrum of the first DBR 302 are tuned to coincide in wavelength, the round trip gain of the laser cavity becomes sufficient to overcome the optical cavity loss and to achieve lasing at that wavelength. This DBR laser tuning method is known as the digital supermode tuning method.
The tuning range of the laser 300 of FIG. 3 may be extended by increasing the number of peaks in the comb-like reflection spectrum of the first DBR 302 and their wavelength range, and correspondingly increasing the spectral wavelength range of the second DBR 304. However, increasing the wavelength range of the second DBR 304 necessitates that it should be longer, thus increasing the length of the lasing cavity, and the closeness of the optical modes within the cavity with respect to wavelength, which may increase the strength of unwanted optical side modes to the lasing mode.
Thus there remains a need for an improved design of DBR laser that provides an enhanced wavelength tuning range, and seeks to mitigate at least some of the problems of prior art tunable laser designs.